Present day commercial lightwave systems use optical fibers to carry large amounts of multiplexed digital data over long distances from a transmit terminal to a receive terminal. The maximum distance that the data can be transmitted in the fiber without amplification or regeneration is limited by the loss and dispersion associated with the optical fiber. In general, dispersion is reduced by using single mode fibers which have minimum dispersion at the operating wavelength and by using frequency low chirp transmitters.
To transmit optical signals over long distances, the lightwave systems in use today include a number of regenerators or repeaters periodically located along the fiber route from the transmit terminal to the receive terminal. Each repeater boosts the weak received signal to compensate for the transmission losses which occurred from the last repeater.
The repeaters currently used in long haul lightwave systems are not fully optical. They detect light photoelectrically, amplifying the resulting current electronically and then use the retimed (regenerated) current to drive a semiconductor laser which converts the electrical signal back into an optical signal. The optical signal is then carried in the optical fiber to the next repeater in the system where the conversion from optical to electrical and back again to optical is repeated again.
In an all-optical transmission system the optical signal, once generated, will be transmitted optically, amplified optically and received for detection at the destination. There will be no intermediate conversion of the signal being transmitted from optical-to-electrical and then back to optical. The use of an optical amplifier to provide direct optical amplification of an optical signal will result in the elimination of the electronic processing procedure and will enable optical communication systems to have repeaters which have higher bandwidths are physically smaller, simpler in design, more efficient to operate and more economical to produce.
One area which can present a problem is that of monitoring the in-service performance and the operating parameters of the optical fiber amplifier when used as a repeater. This is necessary because, as an example, the gain of optical amplifiers can be affected by both environmental effects (i.e. variations of ambient temperature) and changes in system variable (i.e., changes in source wavelength and the polarization of the input signal). Another area of concern when using an optical fiber amplifier as a repeater in an optical communication system is that of being able to send telemetry command signals to the repeater and having the repeater detect the telemetry command signals. Still another area of concern is the need to permit maintenance personnel located at one optical fiber amplifier repeater to communicate with other maintenance personnel located at another optical fiber amplifier repeater.
Thus, in long haul optical fiber transmission systems which include optical fiber amplifier repeaters, a need exists for on-line telemetry to permit the monitoring of system health information, supervisory signals, remote control commands and fault locate data between the terminals and the repeaters. In achieving this, it is imperative that the telemetry channel does not interfere with the traffic on the system.